# Impact of viral telomeric repeat sequences on herpesvirus vector vaccine integration and persistence

## TL;DR

This study shows that telomeric repeats in the turkey herpesvirus genome help it integrate into host chromosomes, which is crucial for its long-term persistence and spread.

## Contribution

The study reveals that telomeric repeat sequences in HVT are essential for genome integration, latency, and virus shedding in vivo.

## Key findings

- HVT integrates into host telomeres, and this integration is severely impaired when telomeric repeats are removed.
- HVT mutants lacking telomeric repeats show reduced viral load and impaired transport to feather follicles.
- Telomeric repeats are crucial for establishing latency and reactivation of HVT in chickens.

## Abstract

Marek’s disease virus (MDV) vaccines were the first vaccines that protected against cancer. The avirulent turkey herpesvirus (HVT) was widely employed and protected billions of chickens from a deadly MDV infection. It is also among the most common vaccine vectors providing protection against a plethora of pathogens. HVT establishes latency in T-cells, allowing the vaccine virus to persist in the host for life. Intriguingly, the HVT genome contains telomeric repeat arrays (TMRs) at both ends; however, their role in the HVT life cycle remains elusive. We have previously shown that similar TMRs in the MDV genome facilitate its integration into host telomeres, which ensures efficient maintenance of the virus genome during latency and tumorigenesis. In this study, we investigated the role of the TMRs in HVT genome integration, latency, and reactivation in vitro and in vivo. Additionally, we examined HVT infection of feather follicles. We generated an HVT mutant lacking both TMRs (vΔTMR) that efficiently replicated in cell culture. We could demonstrate that wild type HVT integrates at the ends of chromosomes containing the telomeres in T-cells, while integration was severely impaired in the absence of the TMRs. To assess the role of TMRs in vivo, we infected one-day-old chickens with HVT or vΔTMR. vΔTMR loads were significantly reduced in the blood and hardly any virus was transported to the feather follicle epithelium where the virus is commonly shed. Strikingly, latency in the spleen and reactivation of the virus were severely impaired in the absence of the TMRs, indicating that the TMRs are crucial for the establishment of latency and reactivation of HVT. Our findings revealed that the TMRs facilitate integration of the HVT genome into host chromosomes, which ensures efficient persistence in the host, reactivation, and transport of the virus to the skin.

Several herpesviruses harbor telomeric repeats (TMRs) at the ends of their genomes. These TMRs are identical to the host telomeres, which protect the ends of host chromosomes from deterioration. One of these viruses is the herpesvirus of turkey (HVT), which is commonly used as a live-attenuated vaccine against MDV but also as a vaccine vector. Our study revealed that HVT efficiently integrates into the ends of host chromosomes containing the telomeres. Removal of the TMRs from the virus genome did not affect virus replication but severely impaired integration. In addition, persistence of the vaccine virus was drastically reduced in the host. Our data also revealed that integration and/or persistence of the virus is crucial for the transport of the virus to the skin of infected animals, where HVT replicates and is shed into the environment. Our study provides important insights into HVT biology and sheds light on the role of the TMRs in virus integration, persistence and shedding.